SAN DIEGO - Federal agents conducted a raid at the Mingei International Museum in San Diego's Balboa Park Thursday morning, culminating a five-year investigation of a suspected smuggling operation.

Three other museums and an art gallery in Southern California also were raided as part of the probe.

The investigation focused on a smuggling pipeline that authorities say funneled looted artifacts to museums and enabled donors to receive inflated appraisals for the pieces in order to file false tax returns, federal documents say.

Search warrants were served on the Los Angeles County Museum of Art, the Pacific Asia Museum in Pasadena and the Bowers Museum in Santa Ana.

Shortly after 8 a.m., federal agents went into the Mingei in Balboa Park, carrying boxes and large bags. About an hour later, officials placed a handwritten sign on the front door announcing the museum was closed for the day. Museum spokeswoman Martha Ehringer said some school group tours scheduled were canceled and visitors were turned away at the door.

According to a federal affidavit, an undercover federal agent bought and then donated stolen Thai archaeological resources to the Mingei on five occasions, starting in June 2006.

The donations were items representative of the "Ban Chiang" culture, called one of the most important prehistoric settlements yet discovered in Southeast Asia.

According to a federal affidavit, the main targets of the investigation included Bob Olson, identified as an alleged art smuggler, and Jon Markell, owner of a Los Angeles Asian art gallery.

As part of the scheme, Markell sold the antiques to the undercover agent for one price and then gave him inflated appraisals of the items, which he could use to take a charitable deduction on his tax return, the affidavit said.

The affidavit details communication between Markell and Mingei Director Rob Sidner as Markell offered the antiquities to the Mingei on behalf of the undercover agent.

It says the undercover agent met with Mingei staff once in a meeting that was recorded and there were six recorded telephone calls, 11 e-mails and nine letters exchanged regarding the museum's acceptance of Thai artifacts.

Donations to the Mingei included a Ban Chiang water buffalo vessel, bracelets, beads, pottery anvils, spindle whorls and rollers. At least one item was appraised at $6,000, even though the undercover agent had paid $1,500 for it, according to the affidavit.

Sidner at one point asked Markell if there were any provenance - or origin - problems with the artifacts, and the gallery owner responded the pieces had all come from his collection and there were no problems.

In fact, since 1961, Thai law has prohibited antiques from being exported from the country unless a license is obtained, according to the affidavit. A Thai official said the government has never given permission to anyone to take excavated antiquities out of Thailand for private sale.

The affidavit says the undercover agent handed Sidner and members of the Mingei museum's curatorial staff copies of the Thai antiquity law in June 2006.

The document says appraisals were prepared by Markell but were electronically signed with another appraiser's signature.

The undercover agent on at least one occasion mailed a copy of the appraisal to Sidner and a letter that stated that Markell had signed another person's name to the document.

The gallery owner told the undercover agent that he had done appraisals and donations of Bank Chiang antiquities for himself and for his relatives, which were all donated to the Mingei, according to the federal affidavit.

Under the search warrant, federal agents planned to identify and catalog all material purchased or obtained with the assistance of Olson or Markell, photos, books and other materials on archaeological resources from Thailand and records dating back to 1998 on transactions involving Olson, Markell or their clients.

The affidavit also allowed the museum's computer records to be searched.